1. Field of the Invention
This invention relates to Infrared transparent conductive thin films for use in electro-optical devices.
2. Description of the Related Art
Many electro-optical devices operating in the infrared require conductive thin films that are extremely transparent. Such devices include liquid crystal cells for beam steering, spatial light modulators, and optical switches for fiber optical communications, transparent conductive electrodes for elector-optical devices such as electo-optical crystals and photo refractive polymers, and long wavelength vertical cavity surface emitting lasers (VCSELs). Conventional transparent conductive oxides such as Indium Tin Oxide (ITO), ZnO and SnO2 cannot fulfill the application specification. These materials are mostly transmissive in the visible, however transmission in the Infrared drops due to free carrier absorption. FIG. 6 is a plot of the transmission values versus wavelength for the prior art ITO and ZnO films on a glass substrate. The ITO film, which is generally the most transparent of the two has a transparency below 60% at 1.31 xcexcm and below 40% at 1.55 xcexcm for a film with a sheet resistance of about 10 xcexa9/xe2x96xa1. For ITO films with a sheet resistance of about 200 xcexa9/xe2x96xa1 the transmission is about 96% at 1.31 xcexcm and about 95% at 1.55 xcexcm. 1.31 xcexcm and 1.55 xcexcm are critical values for fiber-optic communication and long wavelength VCSELs applications. Doped semiconductor wafer or epitaxial layers (Si, Ge, GaAs, InP doped) have limited utility because they do not cover the entire spectral range, have a limited size and are expensive.
As such, there has been a long felt need for Infrared transparent conductive films.
Cadmium Oxide films doped with Indium (CdO:In) have been prepared in the past for flat-panel displays and solar cells which require transparency in the visible and UV region. However, these films are well known for their toxicity, and therefore the prior art has steered away from developing such films. Minami et al reports that CdO:In films have been prepared with a resistively of the order of 10xe2x88x925 xcexa9 cm for flat-panel displays and solar cells, but he states that they are of no practical use because of Cd toxicity. (Minami, Tadatsugu, xe2x80x9cNew n-Type Transparent Conducting Oxidesxe2x80x9d, Transparent Conducting Oxides, Volume 25, No. 8, August 2000, page 38). Further, because these films have a yellow color, which differentiates them from prior art transparent films which are clear in appearance, one skilled in the art would be steered away from using these films for any applications requiring transparent films
Undoped CdO has been prepared by sputtering, MOCVD, and spray pyrolysis (Murthy, L. C. S. and Rao, K. S. R. K., xe2x80x9cThickness Dependent Electric Properties of CdO Thin Films Prepared by Spray Pyrolysis Method,xe2x80x9d Bulletin of Material Science, Vol 22, No 6, pp953-7 (October 1999); Subramarnyam, T.K et al, xe2x80x9cPreparation and Characterization of CdO Films Films Deposited by DC Magnetron Reactive Sputteringxe2x80x9d, Materials Letter, Vol. 35, pp 214-220, (May 1998); Baranov, AM. et al, xe2x80x9cInvestigation of the Properties of CdO Filmsxe2x80x9d, Tech. Phys. Ltr, 23, (10) pp 805-806 (October 1997)). Transmission as high as 85% has been reported in the wavelength range of 600-1600 nm. (see Subramarnyam, p. 218). Baranov et al also reports that the CdO film has been considered for other applications such as solar cells which require transparent electrodes for the visible or UV spectra. In addition, Ferro, R et al has reported fluorine doped CdO films. But the transmissivity of these films are below about 90% with decreasing transmissivity as more Fluorine is added (Ferro, R et al xe2x80x9cF-Doped CdO. Thin Films Deposited by Spray Pyrolysisxe2x80x9d, PMS. State. Sol. (a) 177, P477-483(200). Data plotted in FIG. 6 shows these materials to be similar to prior art undoped ITO and ZnO films.
There is a need for films with lower resistivity which also transmit substantially all of incident IR light. Further, due to losses in systems incorporating these films, transmissivity of IR in the IR spectrum, particularly at 1.31 or 1.55 xcexcm must be at least 99% for practical use of these materials.
A doped Cadmium Oxide (CdO) film has been developed for use in applications requiring conductive, Infrared transparent films. Suitable dopants for the film include any of the Group III elements (i.e. Boron, Aluminum, Gallium, Indium, or Thallium). Additionally, it is believed that suitable films can be prepared using F or H as dopants if properly processed.
The new doped film is substantially transparent to Infrared radiation in the range of between about 0.5 xcexcm and 10 xcexcm showing exceptionally high transmission throughout the IR range (99.8% at 1.31 microns; 99.5% at 1.55 micron; 97% at 3 microns; 93% at 5 microns; and 82% at 10 microns) on sapphire or NGO as glass has very low or no transmission in their frequencies evaluated. These measurement where made using films deposited. The new film has a resistivity of from about 5 to about 10xc3x9710xe2x88x925 xcexa9. Deposited on a glass substrate, the film shows a low sheet resistance of approximately 200 ohm/square.
There are numerous possible applications for such films including liquid crystal cells for beam steering, spatial light modulators, and optical switches for fiber optical communications, transparent conductive electrodes for elector-optical devices such as electo-optical crystals and photo refractive polymers, and long wavelength vertical cavity surface emitting lasers.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings, in which: